You don't have to accept it, do you? You have a contract with Cathay Pacific Airways, and they cannot unilaterally change it. They can try, as they often do - but this is kind of like their proposal in the beginning of a negotiation. They want to put forward a new anchor to the negotiation; one that should not exist. I'm confident that the AOA will nip this in the bud. If I were you, I'd send them a note reminding them that you are not on COS 08, and explaining that wrong contract was sent to you. Simple as that.